This invention relates to ground calcium carbonate compositions having narrow size distributions and to an aging method for producing the compositions.
Calcium carbonate has been used in the paper-making industry for many years to enhance opacity in wet end filling operations as well as coating applications. Optical properties, such as brightness and opacity, of a final paper product are affected by the particle size and particle size distribution of the calcium carbonate compositions used to make them. Generally, the narrower the size distribution of the calcium carbonate composition the better the optical properties of the final paper product.
The calcium carbonates used in the paper making industry come in two principle forms: 1) ground natural calcium carbonates and 2) precipitated calcium carbonates. Sources for natural calcium carbonate include limestone, dolomite, marble, chalk and shells. Precipitated calcium carbonates are generally obtained by the carbonization of calcium hydroxide.
Samples of precipitated calcium carbonates are finer and generally have narrower particle size distributions, and therefore better optical properties, than samples of ground calcium carbonates. For this reason precipitated calcium carbonates have traditionally dominated the paper filler market. In contrast, the theological properties of ground natural calcium carbonates make them better suited for use in paper coating formulations. Such formulations are commercially available in the form of fully-dispersed high-solids suspensions. Ground calcium carbonates have also been used as fillers in the papermaking industry. Unfortunately, however, paper sheets made with ground natural calcium carbonate, while having higher sheet strength, tend to have lower brightness and opacity than those made with precipitated calcium carbonate fillers.
The ability to make ground calcium carbonate fillers with narrower particle size distributions has been limited in the past by the methods used in their production. Conventional methods for preparing ground calcium carbonate compositions for use in the papermaking industry involve the preparation of aqueous calcium carbonate suspensions. Conventional ground calcium carbonate suspensions have a high solids concentration, generally greater than 60 percent by weight. Because high solids suspensions have high viscosities they must contain dispersants to ensure that particles remain dispersed and to lower the viscosity, enabling the suspensions to flow easily for processing. Unfortunately, the presence of dispersants in conventional calcium carbonate suspensions may result in reduced filler retention in papers made from those suspensions.
Once prepared the suspensions are wet-ground to reduce the average size of the filler particles. Finally, the size distribution of the filler particles is narrowed mechanically using classification equipment to remove the fine particles. For example, size classification can be performed by means of a centrifuge, hydraulic cyclones, or by gravitational sedimentation. A disadvantage of methods of production that employ classification equipment is that they add to both the time and cost of paper production by the addition of extra processing steps and equipment.
A method of narrowing the particle size distribution in a sample of precipitated calcium carbonate or ground natural limestone that does not involve the use of classification equipment is heat-aging. Heat-aging, also known as Ostwald ripening, is a process by which crystals of calcium carbonate initially of a higher internal energy state and having a relatively small average particle size undergo a phase transformation by dissolving and redepositing on crystals having a lower internal energy. The heat-aging process requires that the sample be heated to elevated temperatures, generally near 100xc2x0 C., and it has been found that heat-aging will not occur to any significant extent at temperatures below about 40xc2x0 C. or at pH levels less than about 8.5. Unfortunately, conventional heat-aging is a slow, time-consuming, and highly capital intensive process.
Thus a need exists for a simple, efficient, and low-cost method of producing a substantially dispersant-free ground natural calcium carbonate filler composition having a narrow particle size distribution.
This invention is based on the discovery that the size distribution of calcium carbonate particles in a substantially dispersant-free suspension can be narrowed by allowing the suspension to age in the absence of heating.
The present invention provides finely divided ground natural calcium carbonate filler compositions having very narrow particle size distributions and a method for making the compositions. In particular, this invention provides a method wherein a substantially dispersant-free aqueous suspension of calcium carbonate is wet-ground to produce a calcium carbonate composition having a given particle size distribution. The suspension is then allowed to age at temperatures below 40xc2x0 C., resulting in a narrowing of the calcium carbonate particle size distribution.